1. Technical Field of the Invention
The field of the invention is that of optical devices allowing the orientation of an object in space to be measured without contact. There exist various possible fields of applications, but the main application is posture detection for a helmet of an aircraft pilot, thus allowing an image to be projected into his/her visor in exact superposition over the external scene or various systems of the aircraft to be closed-loop controlled onto his/her gaze. The precision sought in such systems is of the order of a milliradian. In the description that follows, the posture of an object is understood to mean its position and its orientation in space.
2. Description of the Prior Art
Various optical techniques exist that allow measurements of orientation to be made on a helmet. Generally speaking, easily-identifiable elements are installed on the helmet which are picked out by a system of cameras. The position of the images of these identifiable elements allows the orientation of the helmet to be determined by calculation.
These elements may be passive or active. The passive elements are illuminated by an external source. For this purpose, retro-reflecting cube corners or retro-reflectors may be used. The optical emitting and receiving devices just need to be placed on the same axis.
These retro-reflector systems are relatively insensitive to sunlight. They may be combined with a point source associated with a sensor array without an optical lens.
In this configuration, the reflector is equipped with a mask that is transmissive in the central part and opaque around the periphery. This mask is applied onto the entry face of the reflector. By way of example, the contour of the mask has the shape of a parallelogram, thus representing the orientation of two fixed directions of the helmet. The orientation of the helmet is calculated by analysis of the shape of the contour projected onto the sensor. The analysis relates to the transitions between the illuminated and dark regions of the reflection received by the sensor. This optical device is simple to implement and has a large depth of field.
FIGS. 1 and 2 illustrate this type of detection system. In FIG. 1, the point source S illuminates a cube corner CC comprising a mask MK. The light beams reflected by the said cube corner are received by two photosensitive sensor arrays disposed in two different planes P1 and P2, close to the source S so as to limit the size of the image. Thus, the points M1 and M2 of the mask have as respective projections, in the planes P1 and P2, the pairs of points K2 and M″1 and M″2. The orientation sought for the sides bounded by the points M1 of the mask MK is obtained by combining the measured orientations of the sides of the contours bounded by the points M′i and M″i of the light reflections on the planes P1 and P2.
FIG. 2 shows one variant of the system described in FIG. 1. In this variant, a shuttering screen EC is placed in the neighbourhood of the source. The projected image of the mask is obtained on a single sensor array disposed in the plane P1, as previously, and also the dark contour of the shuttering screen EC. This contour is bounded in the plane of FIG. 2 by the points e′1 and e′2. The position of the point S0, which is symmetrical with the point S with respect to the apex O of the cube corner, is obtained from the measured positions of the apices of the shadow projected by the shuttering screen. The sought orientation of the sides of the mask MK is then obtained by combining the measured orientations of the sides of the contour of the light reflection on the plane P1 with the measured position of S0.
However this technical solution has certain drawbacks. In the first place, the illuminating point source continuously emits into an angular opening that is sufficiently wide to cover all the possible positions of the reflector. A very large part of the light intensity is therefore always lost, since it does not reach the reflector. This part of the light is, moreover, radiated into the surrounding space, inside and outside of the cabin, thus being detrimental to the stealth of the aircraft.
Furthermore, in the central projection used for determining the orientation of the reflector, the plane of projection is known, but not the position of the centre of projection. In order to eliminate this unknown, two measurements are combined together that are carried out over the contours of two different projected figures obtained with the same centre of projection. The overall uncertainty in the orientation measurement cumulates the error of each of these two measurements.
Lastly, the posture is calculated after the complete image has been acquired and supplied by the image sensor or sensors. The duration of the measurement cycle is limited by the time for addressing an image sensor array, i.e. around 16 ms, which can lead to a measurement error in the case of a high-speed rotation of the mobile object.